You don't want any ferric metal near the compass when you are determining the deviation. If you wear a watch make sure put that arm behind you back and if you wear glasses with metal in the frames take them off. Larger pieces of metal (such as your rifle) need to be at leat 10 meters away and power lines at leat 150 meters. If you are near a street, and you are using a quality compass like an M2 aiming circle or a transit, you will see the compass needle move when a car drives by.
Since you are trying to determine your compass' deviation your determination can never be more accurate than you knowledge of the local variation which you must back out of the total difference that you measure from the true direction to the distant object (use google earth to get this). Go here to get accurate variation infor:
http://www.ngdc.noaa.gov/geomag/declination.shtml
gl
--- On Wed, 11/14/12, Bruce Pennino <bpennino.ce@charter.net> wrote:
From: Bruce Pennino <bpennino.ce@charter.net>
Subject: [NavList] Re: Test your magnetic compass.
To: NavList@fer3.com
Date: Wednesday, November 14, 2012, 3:09 PM
Good idea. However, surveyors worry about
overhead and underground powerlines, large water pipes,water even
moving in pipes, nearby cars/trucks. Maybe all of these influences
cancel, but must consider them.
Bruce
----- Original Message -----
Sent: Wednesday, November 14, 2012 5:27
PM
Subject: [NavList] Test your magnetic
compass.
Byron: I am interested in the land small magnetic compass and how to best
get a fix on land. I have not experience the "how to on land" but will try a
few things to improve its use, let me tell about my thinking.
Most streets
and intersections are 90 degrees and the directions are known. Test the
compass.
My street, runs the same as variation for this area.15 degrees W.
If you stand on the curve and sight up the street than down the street,
reading should be 180&000& across 090&270. If not, it would be
compass in error by the difference. EXAMPLE : sight up 000 down 180 for zero
error or if 000 and 190, the error would be 10 degrees, 5 up & 5down. Also
will the compass return to the same reading or indicate a hang up when
resighting directions. Because of the radiant rule of sixth, the two nearest
LOP’S intersection should be the fix. A far navaid should give the correction
to the compass error. If the far is 6 miles the 12000 yds would cover a right
angle to the line of sight of 200 yds on each side. This allows large position
error so a sight back to the far target should give improved correction of the
systematic error of the two shorter LOP intersection, leaving the random and
uncontrollable error associated with small hand held compass. I have stood at
the corner of my local street, set to variation 15W, and read ooo and 182 with
a repeat of the same. I am using a very cheap compass. Holding it to my cheek
and aligning with my right eye. Try this street test for practice and
information on your magnetic compass. You need not know your street variation
as long as you have a long reciprocal line on each side of you and street
curve or objects to take bearing on.
This is part of an old trick to
correct magnetic compass on a boat. Head to a buoy, read the compass, add 180
degrees, put buoy on your stern and head away, read the compass, it should be
the added 180 this difference gives the amount of compass error. Move magnets
or adjust the compass to ½ the found error. See what your compass
reads!
Let me know what you
think.
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